<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      span {
        display: inline-block;
        width: 250px;
        height: 30px;
        vertical-align: middle;
        line-height: 30px;
        padding-left: 15px;
      }

      .error {
        color: red;
        background: url(./images/error1.png) no-repeat left center;
      }

      .right {
        color: green;
        background: url(./images/right.png) no-repeat left center;
      }
    </style>
  </head>

  <body>
    <input type="text" />
    <span></span>
    <script>
      // 1. 准备正则
      const reg = /^[a-zA-Z0-9-_]{6,16}$/;
      const input = document.querySelector("input");
      const span = input.nextElementSibling;
      input.addEventListener("blur", function () {
        if (reg.test(this.value)) {
          span.innerHTML = "输入正确";
          span.className = "right";
        } else {
          span.innerHTML = "请输入6~16位的英文数字下划线";
          span.className = "error";
        }
      });
    </script>
  </body>
</html>
